The present invention relates to a sheet warp prevention mechanism, employable to an image forming apparatus such as a printer and the like utilizing a so-called continuous-form sheet as a recording medium and including a pair of fixing rollers adapted to be brought into and out of contact with each other, for preventing the continuous-form sheet from being warped when the rollers are brought into contact with each other.
Conventionally known is an electrophotographic method in which a photoconductive material on a rotating drum, i.e., a photoconductive drum, which has been evenly charged, is exposed to light in accordance with an image to be developed for forming a latent image. Toner particles are adhered onto a surface of the drum for forming a visible toner image, and the-toner image is transferred onto a continuous-form sheet having been fed along a predetermined feeding path and fixed thereon at a fixing unit.
Also known is an electrophotographic printer, as an image forming apparatus employing the above electrophotographic method, utilizing a fan-folded continuous-form sheet used in a conventional line printer and the like. The fan-folded continuous-form sheet, simply referred to as "continuous sheet" hereinafter, is arranged to provide, at both side edges, a pair of sprocket holes in predetermined intervals along a longitudinal direction thereof. Further, on the continuous sheet, a plurality of perforated lines are provided along a width direction in another predetermined interval so that the continuous sheet is easily cut into pages.
In the conventional image forming apparatus such as the above arranged electrophotographic printer, the method for fixing a toner image having been transferred onto the continuous sheet is a so-called heat roll fixing method in which the continuous sheet is pressurized by means of a heat roller which is arranged to be heated up to a predetermined high temperature, and the toner image is fixed by heat from the heat roller.
A heat roll fixing unit for the above fixing method comprises a pair of rollers arranged to be positioned in parallel and to be brought into and out of contact with each other, and the continuous sheet is fed through the rollers. One of the rollers is arranged to be heated up, referred as "heat roller" hereinafter, by a predetermined heating member such as a halogen lamp, not shown accommodated within the heat roller. In other words, the continuous sheet is pressurized with heat when passed between the heat roller and the other roller, referred to as the "press roller" hereinafter, and therefore, toner image is fixed on the continuous sheet. The press roller is made of an elastic material and is arranged to be compressed in a radial direction thereof when pressurized by the heat roller. By employing the above Fixing method, it becomes possible to fix the toner image at high speed with high thermal efficiency. Usually, the heat roller is driven to be rotated so that the continuous sheet is fed along the predetermined feeding path at the fixing unit.
When the heat roll fixing method is employed in the electrophotographic printer using the above continuous-sheet as the recording medium, a problem arises in that when the continuous-sheet is held between the rollers without feeding, the same portion on the continuous sheet is burned or blistered by the heat applied from the heat roller. In order to overcome the problem, a fixing unit has been proposed, wherein one of the rollers can be retracted from the contacted state for preventing the continuous sheet from being contacted with the heat roller when the printing operation is not to be executed, see, for example, Japanese Patent Provisional Publication No. HEI 1-163790.
As shown in FIG. 1, this type of fixing unit 10 comprises a heat roller 11 and a press roller 12 between which a continuous sheet 20 is fed along a direction as indicated by an arrow "A". The heat roller 11 is supported by an arm member 13 which is arranged to be rocked as indicated by an arrow "B" by means of a well-known rocking mechanism. For example, as shown in FIG. 2A, it may be constructed that a rotating shaft 11-1 of the heat roller 11 is connected to a cam follower 101 arranged to be movable with a cam 102 having a long radius portion 110 as well as a short radius portion 120. The cam 102 is arranged to be driven to rotate by means of a driving source 103, such as a motor. The rotating shaft 11-1 of the heat roller 11 is inserted into an oval hole 11-2 provided on a chassis 11-3 of the printer, so that the cam follower 101 is vertically moved in accordance with rotation of the cam 102. Therefore, the heat roller 11 is rocked along the "B" direction. When the cam follower 101 is contacted with the long radius portion 110 of the cam 11C, the rollers 11, 12 are brought out of contact from each other. On the other hand, when the cam follower 11B is contacted with the short radius portion 120, the rollers 11, 12 are brought into contact with each other. When the arm member 13 is upwardly rocked, the heat roller 11 is upwardly rocked too, and the heat roller 11 and the press roller 12 are brought out of contact from each other.
The press roller 12 is upwardly biased toward the heat roller 11 by means of a predetermined biasing member such as a spring 14, so that the heat roller 11 downwardly pushes the press roller 12 against the biasing force of the spring 14 when the heat roller 11 and the press roller 12 are brought into contact with each other, i.e., the heat roller 11 is located at an operating position, and therefore, a predetermined pressure force is generated between the rollers 11, 12.
In this type of printer, an interval between an operating position of the fixing unit 10, at which the rollers 11, 12 are brought into contact with each other, and a transfer position of the photoconductive drum (not shown), at which the toner image is transferred to the continuous sheet, is arranged to be substantially similar to an interval between two adjacently located perforated lines of the continuous sheet 20, i.e., length of one page so that a fixing operation for one page is simultaneously finished with a transfer operation for the succeeding page. In order to stably feed the continuous sheet 20, there is provided a tractor 2 between the fixing unit 10 and the photoconductive drum.
The tractor 2 and the fixing unit 10 are driven to feed the continuous sheet 20 by the same driving source 3, such as a step motor, since the continuous sheet is to be synchronously fed by the fixing unit 10 and the tractor 2.
A gear 3B is driven to be rotated along a direction indicated by an arrow "C" by an output gear 3A provided within the driving source 3, and a tractor driving system 4 including a pair of gears 41, 42, which are respectively arranged to be rotated along directions indicated by arrows "C1" and "C2", is driven through the gear 3B so that a tractor belt 21 on which the continuous sheet 20 is located is moved along a direction indicated by an arrow "D" by means of a pulley 42A which is coaxially arranged in an extremely close positional relationship with the gear 42A. On the other hand, a heat roller driving system 5 is simultaneously driven through the gear 3B. The gear 3B and 41 are, in fact, coaxially provided with each other in an extremely close positional relationship though the gears 3B, 41 are illustrated in a separated state in FIG. 1. In FIG. 1, positional relationships among each of elements are schematically illustrated for better understanding. The heat roller driving system comprises a timing pulley 51 which is coaxially provided with the gear 41 in an extremely close positional relationship, a timing belt 52 arranged to be driven to move by the timing pulley 51, another timing pulley 53 to be rotated through the timing belt 52, a gear 54 to be rotated by the timing pulley 53, an idle gear 55 to be brought into engagement with the gear 54 and arranged to be coaxially provided with the rocking center of the arm member 13, and a heat roller gear 56 which is coaxially provided with the heat roller 11 and arranged to be brought into engagement with the idle gear 55.
In other words, the heat roller 11 is arranged to be rocked in accordance with the rocking operation of the arm member 13, and the heat roller driving system 5 for driving the heat roller 11 are arranged to transmit drive force from the driving source 3 toward the heat roller through the idle gear 55 which is coaxially provided with the rocking center of the arm 13.
In this above-described arrangement in which the heat roller 11 is rocked by means of the arm member 13 and driven to be rotated through the idle gear 55 which is coaxially provided with the rocking center of the arm member 13, however, a problem arises in that the continuous sheet 20 between the photoconductive drum and the fixing unit 10 is slightly warped upwardly when the heat roller 11 having been separated from the operating position is rocked and brought into contact with the press roller 12.
The reasons for the above-described sheet warp are as follows: first, the idle gear 55 is connected to the driving source 3 as well as the tractor driving system 4 when the heat roller 11 is rocked and located at the operating position. Rotary resistance as a load, in this state, becomes large when the arm 13 is rocked. Therefore, the heat roller gear 56 which is constantly brought into engagement with the idle gear 55 is rotated in the opposite direction of the sheet feeding direction. That is, the heat roller 11 which is coaxially provided with the heat roller gear 56 is slightly rotated in the opposite direction, so that the continuous sheet is reversely fed in accordance with the reverse rotation of the heat roller gear 56, and accordingly, the continuous sheet 20 is slightly warped upwardly between the photoconductive drum and the fixing unit 10.
Further, as the heat roller 11 is rocked and located at the operating position, i.e., brought into contact with the press roller 12, the pressure force generated between the rollers 11, 12 becomes larger than a predetermined value since the press roller 12 is slightly compressed by the heat roller 11 and the spring 14 is slightly pushed downwardly against elastic force therof. Therefore, if the pressure force becomes larger than the predetermined value, rotary resistance of the heat roller gear 56 extremely increases and the heat roller 11 can not be rotated even when the arm member 13 is rocked. As a result, both the arm 13 and the heat roller gear 56 are slightly moved in a downward direction, and the idle gear 55 is relatively moved in an upward direction. In other words, the idle gear 55 is operated as if it were a planetary gear arranged to be rotated around the heat roller gear 56. Since the idle gear 55 is undesirably rotated in a direction indicated by an arrow "E", i.e., a direction corresponding to the sheet feeding direction, in accordance with the rocking operation of the arm member 13, and the tractor 2 is slightly driven to feed the continuous sheet 20 toward the fixing unit 10. In other words, the tractor 2 is undesirably driven by the rotation of the idle gear 55 to feed the continuous sheet 20 in the sheet feeding direction, rather than solely by the driving source 3. Accordingly, the continuous sheet 20 is fed by the tractor 2 without a feeding operation of the fixing unit 10, so that the continuous sheet 20 is further upwardly warped.
Further, when the tractor 2 is driven to feed the sheet in the sheet feeding direction by the rotation of the idle gear 55 as described above, so-called play among the elements in the heat roller driving system 5, such as backlash between each of elements provided from the gear 3B to the heat roller gear 56 through the idle gear 55, are shortened by movement of each of driven side elements 1000 as shown in FIG. 2B. On the other hand, play among the elements in the tractor driving system 4, i.e., elements provided from the gear 3B to the tractor 2, are shortened by movement of each of the driving side elements 2000 as shown in FIG. 2C. Character "a" in FIGS. 2B and 2C indicates a direction corresponding to the sheet feeding direction. As a result, when the driving force form the driving source 3 is applied, the tractor 2 is immediately started to feed the continuous sheet 20, however, the heat roller 11 is not driven to rotate so immediately since it is necessary to elapse a predetermined period of time for shortening the play in the heat roller driving system 5 in the sheet feeding direction. Therefore, the continuous sheet 20 is further upwardly warped.
Consequently, when the continuous sheet 20 is upwardly warped by the above-described reasons, the warped continuous sheet 20 is contacted with an element 6 such as a chassis and the like provided above the feeding path of the continuous sheet 20, and the toner image on the continuous sheet 20, which has not been fixed yet, is disturbed by the element 6. Further, since the continuous sheet 20 is fed into the fixing unit 10 in a warped state, wrinkles occur thereon.